1. Field of the Invention
The present invention relates to a method and to an apparatus for processing high-ash coal slurries by flotation of a sludge in the cells of flotation units, particularly for processing gas coal and open-burning coal which are difficult to float.
2. Description of the Prior Art
In hard coal processing, foam flotation is usually employed for the production of coal concentrate from high-ash coal slurries, particularly in the grain size range below 0.5 mm. Because of the increasing production of coal slurry, it is gaining greater and greater significance. The increasing production of coal slurry, as a result of increasing proportions of fine and finest material in the raw material, is a result of increasing mechanization in mining.
As a result of this development, there is a necessity to improve the known methods and apparatus for processing high-ash coal slurry, particularly foam flotation grading, and to automate the same, and to automate the same in consideration of the optimum operating points. However, relatively slight, carbonized "younger" bituminous coal whose slurry, moreover, is high in unsolidified, extremely finely-distributed argillaceous minerals which have a flotation-inhibiting effect, present particular difficulties.
As is known in the art, foam flotation is based on the method of dispersing gas or, respectively, air bubbles in the sludge liquid in order to therefore equip coal and middlings particles with the required buoyancy so that a surface foam arises which is high in coal components and low in attle or, respectively, ash components. Since the creation of gas bubbles and their proper distribution (among other things) are a function of time, if frequently occurs that no sufficient generation of gas bubbles occurs in the first cell of a flotation system. This is only achieved, to a satisfactory degree, in the second and following cells. Although this disadvantage can be countered with an increased plurality of cells, the capital expense and the energy and space requirements occasioned rise to a considerable degree.
It is further known that, given a high concentrate component of the sludge, the flotation material demonstrates a tendency to rise quickly and, as a closed mass, uncontrollably, to the top, whereby it entrains undesired components of argillaceous and shale minerals. Therefore, the cleans deteriorate. Thereby, the disruptive influence of flocculents becomes noticeable at the same time, the flocculents being employed in the pre-connected coal washing in order to cause sludges from washing processes in thickners to settle out with a high specific clarification surface mode. In a known manner, they effect an agglomeration of fine solids particles into larger structures with a higher sinking rate. In the following flotation process, however, the increase of the sinking rate causes a continuing disruption of the grading effect because the generally-insufficiently selective flocculents agglomerate attles particles, middlings and coal particles, as well as into undesired mixed structures. This leads to an increase of the coal component in the attles.
Thereby, further disadvantages result that flotation cells are generally connected in series, whereby the concentrate is stripped off in every cell, whereas the attles, which are contained in the respective sinks, traverse all cells. Therefore, an error propagation occurs, particularly regarding the flocculents. Thereby, optimally-set flotation cells having flocculent-free charges (laboratory conditions) operates significantly more favorably than operating systems in which all, partially counter-productive factors, have heretofore not been able to be taken into consideration. In the operating systems, it is particularly fluctuating charge amounts which lead to fluctuating selectivity and, therefore, to poor production results.
In the prior art, the known difficulties lead to multifarious solutions, for example, to flotation systems in which the attles of the after-flotation and/or the concentrate components, particularly of the first cells, were multiply retreated. In practice, however, none of the known flotation systems achieve the desired results over a long term. In particular, the coal component in the attles is too high.